(i) Field of the Invention
This invention relates to a piezoelectric single crystal for use in ultrasonic vibrators etc., particularly surface acoustic wave elements, and also to a surface acoustic wave element employing the single crystal.
(ii) Brief Description of the Prior Art
At present, various sorts of piezoelectric single crystals are known. Among them, typical single crystals are quartz, LiNbO.sub.3, LiTaO.sub.3, Ba.sub.2 TiSi.sub.2 O.sub.8, etc. These single crystals are properly used according to purposes as their characteristics have both merits and demerits.
The quartz crystal is applied extensively to piezoelectric vibrators etc. Although the temperature characteristic of the quartz crystal is good, the piezoelectric activity is comparatively low, and the electromechanical coupling factor of the thickness vibration as is a constant representative of piezoelectricity is approximately 10%. Regarding the surface acoustic wave properties, the temperature coefficient of delay time is substantially 0 ppm/.degree.C. and is good, but the electromechanical coupling factor k.sub.s.sup.2 is as small as .about.0.1% and is poor. On the other hand, the LiNbO.sub.3 crystal exhibits a high piezoelectric activity, but it has the disadvantage of a poor temperature characteristic. That is, although the electromechanical coupling factor of the thickness-shear vibration of an X-cut plate is as great as 60-70%, the temperature coefficient of the resonance frequency is as great as 70-90 ppm/.degree.C. Regarding the surface acoustic wave properties, the electromechanical coupling factor is approximately 5% and is excellent, whereas the temperature coefficient of delay time exhibits a large value of 70-90 ppm/.degree.C. unpreferably.
The LiTaO.sub.3 and Ba.sub.2 TiSi.sub.2 O.sub.8 single crystals have characteristics intermediate between those of the aforecited two crystals. The electromechanical coupling factor of the thickness-shear vibration of the LiTaO.sub.3 single crystal of an X-cut plate is approximately 47%, and the temperature coefficient of the resonance frequency is approximately 11 ppm/.degree.C. The electromechanical coupling factor and temperature coefficient of surface acoustic wave (SAW) delay time are approximately 0.7% and approximately 22 ppm/.degree.C., respectively. On the other hand, the electromechanical coupling factor of the thickness-shear vibration of a bulk in the Ba.sub.2 TiSi.sub.2 O.sub.8 single crystal of a (110)-cut plate is approximately 28%, and the temperature coefficient of the resonance frequency is approximately 33 ppm/.degree.C. The electromechanical coupling factor and temperature coefficient of delay time are approximately 1.5% and approximately 50 ppm/.degree.C., respectively.
As materials for SAW devices, besides the piezoelectric single crystals referred to above, there are employed ceramics, piezoelectric thin films etc. The SAW devices employing the ceramics or the thin films, however, have the difficulty that dispersions and secular changes in the characteristics thereof are great.
In case where the temperature coefficient of SAW delay time is great, fluctuations in the characteristics of an SAW device employing the piezoelectric single crystal as are ascribable to temperature changes become great unpreferably. In case where the electromechanical coupling factor is small, a broad-band SAW device employing the piezoelectric single crystal has the disadvantage that the insertion loss increases.
Further, the LiNbO.sub.3 and LiTaO.sub.3 crystals are ferroelectric materials, resulting in the disadvantages that the poling must be carried out for their uses in SAW elements.
On the other hand, the Ba.sub.2 TiSi.sub.2 O.sub.8 single crystal having the mineral designation of "fresnoite" possesses, as stated previously, the comparatively good characteristics that the electromechanical coupling factor of surface acoustic waves is somewhat high and that the temperature coefficient of delay time is somewhat small. Further, the Ba.sub.2 TiSi.sub.2 O.sub.8 single crystal has the merits that it does not require the poling because it is not a ferroelectric material and that it can be used even under a high temperature condition because its melting point is approximately 1,400.degree. C. Since the Ba.sub.2 TiSi.sub.2 O.sub.8 single crystal has a dielectric constant of at most 15, it has a small electrical capacity, and it can be used up to high frequencies when applied to SAW elements.
In general, characteristics desired for a piezoelectric single crystal to be used in an SAW element are at least 0.6% in the electromechanical coupling factor k.sup.2 and at most 50 ppm/.degree.C. in the temperature coefficient of delay time. The latter is more favorable as it is smaller.
The following references are cited to show the state of the art; (i) Japanese Published Unexamined Patent Application No. 48-98397, (ii) Japanese Published Unexamined Patent Application No. 51-103792, (iii) Japanese Published Unexamined Patent Application No. 53-68189.